Create debug only SkSingleOwner

include/gpu/GrContext.h

Index: include/gpu/GrContext.h
diff --git a/include/gpu/GrContext.h b/include/gpu/GrContext.h
index fdc6167b14f346358966e455292c2ffe37cc89e2..61dab7646fa12ad5f09d7d1f201ea7b88a8a99d9 100644
--- a/include/gpu/GrContext.h
+++ b/include/gpu/GrContext.h
@@ -390,6 +390,9 @@ private:
     SkMutex                         fReadPixelsMutex;
     SkMutex                         fTestPMConversionsMutex;
 
+    // In debug builds we take a debug mutex to guard against improper thread handling

[mtklein, 2016/01/06 15:08:27]

I'd put it right here.

[joshualitt, 2016/01/06 15:09:47]

This will end up being used by a number of Gr* classes and SkGpuDevice.  I can
make it GrSingleOwner and move it to its own header.

+    SkDEBUGCODE(mutable SkSingleOwner fSingleOwner;)
+
     struct CleanUpData {
         PFCleanUpFunc fFunc;
         void*         fInfo;